GULLANE, Scotland — Carl Pettersson’s golf bag was a little bit lighter. And the burly Swede didn’t stand quite so tall Thursday in the British Open.

All because of his putter.

Pettersson has used a long putter that he anchors to his chest since he was at North Carolina State. For the first time in a major — for the first time that it really mattered — he switched to a conventional putter on Thursday at Muirfield.

The USGA and R&A adopted a new rule in May that will ban the anchored stroke starting in 2016. Pettersson was among those who were strongly opposed to the rule, and he was one of the players singled out as how the ban might affect a career. The broom-handled putter is all he has used as a pro. He has put in thousands upon thousands of hours practicing with it. But he also realizes change is coming.

‘‘Since this rumor of the ban came out last year, I grabbed a couple of regular putters — regular length — and put my hand on it and gripped down on the shaft,’’ he said. ‘‘And it felt pretty good. It was not serious practice, but I hit a few. It’s essentially the same grip with the right hand, so I just had to change the left hand. It’s the same motion I’ve been doing with a long putter. It’s just not anchored.’’

Pettersson first used the conventional putter in the final round of the John Deere Classic, where he was at the bottom of the pack. He shot 70 and tied for 54th.

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‘‘It was nerve-racking at the John Deere,’’ he said. ‘‘If it hadn’t gone well, I would be back to square one. But I did nice. I holed all the putts you’re supposed to hole. I didn’t run the tables, but it was nice.’’

The real test was the British Open on Thursday, when he essentially started out in a tie for the lead. And he had no complaints, especially after making a 30-foot birdie putt on the opening hole. More pleasing was that he didn’t three-putt in his round of 74.

Is that the end of the long putter forever?

‘‘I can’t say forever. Well, we only have two more years,’’ Pettersson said.

Tales of a teenager

The last five days must feel like a blur to 19-year-old Jordan Spieth. He won the John Deere Classic on Sunday for his first professional win, which qualified him for the British Open. He flew on a charter overnight to Scotland. He saw Muirfield for the first time.

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And then he went out Thursday and had one of only 13 rounds in the 60s.

Perhaps even more impressive is that Spieth had only one bogey, dropping a shot on the par-3 fourth hole. He birdied two of the par 5s and added another birdie on the short par-4 third hole.

Spieth started the season without status on any tour and now has a PGA Tour title, an exemption through 2015 on tour, and more than $2 million.

‘‘There’s even less pressure than there was before,’’ Spieth said. ‘‘I kind of accomplished more than I’d thought possible this year.”

Minus two

Two players withdrew, but only after their rounds started.

Peter Hanson, who has been coping with a bad back, pulled out after five holes. Louis Oosthuizen, playing with Tiger Woods and Graeme McDowell, withdrew with a recurring neck injury after a bogey on the eighth hole. He was 4 over for his round. It was the second straight major the South African couldn’t finish.

‘‘I’m bitterly disappointed to have to pull out of the Open, and it looks likely now I am going to have to take some time off and give my neck the rest I’ve been told it needs,’’ he said.